1. Field of the Invention
This invention relates to circuits for generating television (TV) synchronization signals, including horizontal line and vertical frame synchronization. More particularly, the invention relates to circuits for generating video synchronization signals compatible with National Television Standards Committee (NTSC) standards requiring that six equalizing pulses precede and follow a vertical synchronization pulse broken by serrations into six pulses. Specifically, the invention relates to a circuit for accepting a horizonal frequency input signal and a vertical frequency input signal, both of which frequencies may be variable over a wide range, and generating a composite video synchronization signal complying with the NTSC standards.
2. Description of the Prior Art
Integrated circuits (ICs) are now available for accepting a fixed master reference frequency and generating a NTSC synchronization signal having fixed horizontal and vertical frequencies. National Semiconductor, for example, manufactures a TV camera sync generator IC, part No. MM 5320, which is a p-channel enhancement-mode MOS device accepting a master clock of 2.04545 MHZ or 1.260 MHZ, depending on the logic state applied to an input pin.
ICs are also available that interface to a microprocessor address bus and may be programmed to generate independent horizontal and vertical synchronization signals of various frequencies. Motorola, for example, manufactures a cathode ray tube (CRT) controller (CRTC) IC, part No. MC 6845, that is compatible with its MC 6800 line of microprocessor ICs.
Although CRTCs typically allow designers to program a wide range of horizontal and vertical synchronization rates, the signals generated by the CRTC are not compatible with the NTSC specifications for serrations and equalization pulses during the vertical interval. This leaves designers with a difficult choice. On the one hand, they may ignore the NTSC standards and rely on the CRTC output alone for video synchronization signals. This choice places constraints on the type of monitor that may be synchronized to the CRTC signals. Picture quality is especially important in such fields as psychological and vision testing where the attributes to be tested, such as patient's ability to discriminate large areas of low contrast, must be precisely measured independent of variations in quality of the CRT image generated by the test instrument. On the other hand, circuit designers may generate NTSC standard signals without using a CRTC. This choice eliminates the flexibility of the CRTC, since prior art circuitry is capable of providing proper serrations and equalization pulses only at fixed horizontal and vertical rates. Flexibility is useful in selecting the highest horizontal rate that a given CRT monitor may handle, in order to maximize picture resolution. Flexibility is also needed in the field of psychological and vision testing since it is desirable to test a subject's responses to a wide range of visual stimuli, including variations in vertical frame rates and number of horizontal lines per degree of viewing angle, and horizontal lines and vertical bars of various width and spatial frequency.